I have used Auto-Rx and its a great product. Had a 78 volare with a slant six and it brought the compression back up and gave me 3 more mpg on the road. Application as specified by Frank and was highly pleased.
oh, and what blazer LT said too;
We don't use any Group III base oils in our fully synthetic race oil
formulas. We actually found that a blend of Group IV and Group V works
the best for durability and power. We've done extensive base oil testing
since 1999 when we began having our own oil blended, and a blend of
group IV and group V has always proven to perform the best in our NASCAR
The above is from Joe Gibb's racing, one of the most successful oils in racing right now. Now the conditions these oils are used in are nothing like any of us experience, however it goes to show the superiority of Group IV and V base oils for high end applications.
The reason why AutoRX works so well in these older engines is that they are full of deposits from running Group I, solvent refined oils. If you run ARX in an engine thats used a high end sylube there is very little stuff for the ARX to clean up.
By the way, ARX works just fine with PAO/Ester oils. I ran some with Amsoil in an old motor just to see and it cleaned up nicely. Of course I don't know if this is partially a result of the esters in the Amsoil formulation or strictly from the ARX....